If you are looking for homework 3, here is a link to it.
If you are looking for the makeup exam answers, here is a link to them.
If you are looking for homework 2, here is a link to it.
This repository implements a toy version of the DES algorithm that encrypts 8-bit blocks with 10-bit keys and performs a total of two rounds. It is not particularly secure, but it is useful for educational purposes.
A demonstration application is provided that uses the DES implementation to "securely" send a file over a TCP socket.
To build the code, use Bazel:
~$ bazel build ...
All the executables will be in the bazel-bin/transfer directory.
To run the code, first start a server:
~$ ./server 1337 ky
The server takes two arguments. The first is the port to listen on, and the second is the two-character encryption key to use for received files.
Next, we can use a client to send a file to the server.
~$ ./client 127.0.0.1 1337 ky secret_furry_porn.rar
The client takes four arguments. The first is the IP address of the server, the second is the server port, the third is the two-character encryption key to use, and the fourth is the name of the file to send. The encryption key must match the key used by the server.
If all goes well, you should see some messages from the server, and a file by the same name as the one sent will be written to the server machine.
The implemenation of the code is fairly simple. The server can only handle a single TCP connection at a time. The client connects to the server, and encrypts/sends the file data block-by-block. The server uses the same process on the other end to decrypt the data.
The code uses a simple ECB approach for sending arbitrary-length data. This is not very secure, but it is easy to implement.
Additionally, a single fixed-length header is exchanged by the server and client before the actual transfer starts. The header contains the name of the file, and the size of the file. It is not encrypted.
The algorithm used is a 2-round Feistel cipher that operates on blocks of a single byte. The round function uses 2 S-boxes which each take a 4-bit input and produce a 2-bit output. Each subkey is 8-bits, and generated each round from a 10-bit master key. Decryption is implemented as encryption in reverse, with the subkeys applied in the opposite order.
I have performed differential cryptoanalysis on this algorithm, the results of which are available here.
Additionally, I have a solution to the last problem on the homework here.